Especially since the advent of the Montreal Protocol severely limiting the use of CFC (chlorfluorocarbon) and other halogenated hydrocarbon blowing agents, frothed foams have become increasingly important. By the term "frothed foam" and similar terms is meant a cellular foam product the cells of which are formed by the mechanical incorporation of inert gas, particularly air, nitrogen, carbon dioxide, or argon, into a curing polymer system, with or without the aid of small amounts of blowing agents of the physical or chemical types. Froth foams have been prepared from polymer systems such as SBR latex, PVC plastisol, and polyurethane, to the latter of which the present invention pertains.
Polyurethane froth foams have been used for numerous years, for example in the preparation of foam-backed industrial carpet and carpet underlay. See, e.g., "Mechanically Frothed Urethane: A New Process for Controlled Gauge, High Density Foam", L. Marlin et al., J. CELL PLAS., v. 11, No. 6, November/December 1975, and U.S. Pat. Nos. 4,216,177; 4,336,089; 4,483,894; 3,706,681; 3,755,212; 3,772,224; 3,821,130; and 3,862,879, which are herein incorporated by reference.
In the processes disclosed in these references, the polyurethane reactive components: the isocyanate ingredients (A-side), and polyol ingredients (B-side) are each stored in separate, often heated, and sometimes aerated holding tanks. The two components are then metered into a standard froth foam mixing head at low pressure, to which is also fed a supply of compressed air. The mix head contains mixing blades or similar devices moving at high speed, which whips air into the reacting mixture to produce a foam having a consistency not unlike shaving cream or whipped cream.
Due to the intensive mixing which occurs in the froth foam head, as well as the air normally introduced either intentionally or unintentionally into the B-side holding tank, premixing of the reactive foam ingredients has not been considered necessary. Mixers such as those from Hobart or Oakes, of rather conventional construction, have been thought sufficient to provide thorough mixing of foam-forming ingredients. Even more thorough and efficient mixers include stator cylinders containing multiple rows of pins within which revolves a rotor also carrying multiple rows of pins which can rotate between the stator pins. Such mixers are available, for example, from Lessco Corp., Dalton, Ga.
The froth foam is allowed to exit the mixer onto a conveyor belt on which, for example, a release sheet or carpet backing travels, is leveled with the aid of a roller or doctor blade, and generally passed through a curing oven or heated with radiant energy to cure the foam. For some uses, the foam is conveyed through a large diameter hose to the point of application. For many applications, for example carpet underlay, considerable amounts of fillers such as calcium carbonate or alumina trihydrate are added to the B-side to increase the density and load bearing capacity of the foam.
Despite representing standard industry practice for many years, the processes previously described suffer from numerous drawbacks. For example, the change in ambient temperature which may occur between day-shifts and night-shifts or even between the morning and afternoon of the same shift can cause differences in the rate of the urethane polymerization reaction. Changes in atmospheric moisture can also affect the process as can changes in conveyor belt temperatures, etc., caused by continuous running of the process. In the past, changing processing chemistry past merely adjusting A-side/B-side ratios has required halting the process, adjusting the A-side and/or B-side ingredients in the holding tanks, and restarting the process. However, in most cases, the frothing head, and foam conveyor hoses when used, must be cleaned out. The result is loss of manufacturing time which increases cost of the product. In U.S. Pat. No. 4,925,508, for example, is proposed a disposable polyethylene or polystyrene pre-expansion chamber designed to partially reduce down-time.
In the manufacture of filled froth foam, further problems arise. In commercial processes, fillers such as calcium carbonate or alumina trihydrate are added to the B-side (polyol) in quantities up to 300 parts per 100 parts polyol. The filler and polyol components are intensively high-shear mixed, and transferred to a holding tank which is either unstirred or stirred with but modest agitation. Air may be incorporated into the filler/polyol to aid in the froth foaming process in addition to air supplied at the froth foam head, or may be "unintentionally incorporated" due to air entrained in the filler or incorporated from the head space above the polyol during high speed mixing. Once in the holding tank, however, entrained air tends to rise to the top while filler tends to settle to the bottom. There may be more than a two-fold difference between the B-side density at the bottom of the tank and the top of the tank, i.e., 6 lbs/gal at the top and 14 lbs/gal at the bottom. Since the pumps supplying the froth foam head are positive displacement pumps, not only does the density of the product change over time, but the polymerization chemistry changes as well due to the variation in polyol content of the B-side caused by movement of air and filler.
To counteract the difference in density, some processes link the low pressure positive displacement pumps with mass flow devices which measure mass flow rather than volume flow and adjust volume flow accordingly in a closed loop process. While such measures maintain density, they do not maintain chemical stoichiometry, but rather can adversely affect stoichiometry, since the less dense B-side, the volume of which the closed loop process will cause to increase, may already contain a higher weight percent polyol than that desired.
Also important in filled systems is the phenomenon of B-side viscosity increase over time. Over time, the filler/polyol mixture increases considerably in viscosity, perhaps due to greater wetting of the filler surfaces with polyol. It is not uncommon for the viscosity to increase from 2000 cps to 4000 cps over a time of two hours, for example. The increased viscosity reduces pumping efficiency, and more importantly, adversely affects the frothing operation. The result of this and the foregoing factors make continuous production problematic. It is not uncommon for production to be halted every few hours to adjust process parameters, with the deleterious effects on process time previously described.
At times, it is desirous to provide a froth foam product which is multilayered, for example a first layer of lower density and higher resiliency and a second layer of higher density and lesser resiliency. In the past, production of such products has met with but limited success. At the exterior of the first produced foam surface, the froth exhibits coalescence, forming relatively large cells. Since the second froth foam layer, like the first, does not exhibit the expansion typical of blown polyurethane foams which might be sufficient to force the expanding polyurethane into the surface of the first layer, a second froth foam layer does not adhere well to the first layer, resulting in the potential for delamination during production and/or use.
It would be desirable to provide a process for the production of polyurethane froth foam in which the polyurethane stoichiometry can be adjusted on the fly, rather than requiring shut down. It would be further desirable to provide a process for the preparation of filled polyurethane froth foam in a consistent and reliable manner without resorting to use of mass flow meters and other devices. It would be yet further desirable to provide a process for producing polyurethane froth foam wherein multiple layers of foam may be successfully applied. It is further desirable to provide a process where uniform froth foam can be produced, even at low density.